Background: Prior biomechanical studies of transforaminal lumbar interbody fusion were primarily focused on various posterior instrumentation options, comparison with other fusion techniques, and cage positioning inside disc space. Few studies investigated the biomechanics of various cage designs in terms of construct stability.
Methods: Twelve lumbar motion segments were used in this study. The experimental procedure has two steps: multidirectional flexibility test and cyclic test. In the multidirectional flexibility test, all twelve specimens were tested following intact and five different cages (straight or banana shaped). The straight cages had biconvex or flat profile. In the cyclic test, the twelve specimens were randomly divided into two groups for biconvex and flat cages. Three thousand cycles in axial torsion, lateral bending and flexion extension were applied sequentially and cage migration was measured.
Findings: On average, the cage and posterior fixation reduced the range of motion of the intact condition by 40%, 69% and 75% in axial torsion, lateral bending and flexion extension, respectively. There was no statistical difference in construct stability among all five cages. The cage migration (biconvex vs flat) under cyclic loading was less than 0.2mm and no statistical difference was found.
Interpretation: The experimental results suggest that the geometry of cages, including shape (banana or straight), length, and surface profile (biconvex or flat), does not affect construct stability when the cages are used in conjunction with posterior fixation. With posterior fixation and surface serration, cage migration was minimal under cyclic loading for both biconvex and flat cages.